that is more affordable now than ever before, and is used to produce electricity for a wide variety of residential and commercial uses. This electricity produced from sunlight will be a key part of our journey toward a sustainable energy future.
Solar power is a good way to help the environment, because we reduce or entirely eliminate fossil fuel use, and we also save money, because the electricity generated by solar systems is considered to be free once the systems are in place.
How can solar energy be used to produce electricity?
Human ingenuity has developed two different ways how to harvest the energy of the sun and turn it to the most needed electricity.
The first type, solar thermal system generates electricity indirectly by capturing the heat of the sun to produce steam, which runs a turbine that produces electricity.
The second type is solar photovoltaic system, also referred to as a solar panel array, which produces electricity directly from the sun’s light through the series of physical and chemical reactions, also known as the photovoltaic effect.
Let’s have a look at each of these systems more in detail.
How does solar thermal generate electricity?
You might be familiar with the look of the solar thermal technology from 2013, when pictures of the largest solar plant in Mojave Desert, California, made their debut in the press. The array of 170,000 heliostats, looking like large mirrors, in the Ivanpah Valley powers over 140,000 households. Each of these mirrors is tracking the sun’s movement across the sky to reach the highest efficiency¹.
Mirrors are one of the main components of the solar thermal system. They reflect sunlight onto a receiver containing a heat-transfer fluid, which is warmed up by the sun. The fluid used for this purpose is often molten salt, a mixture of 60% sodium nitrate and 40% potassium nitrate. Molten salt has a great capacity to store and transfer heat². This allows the storage of the heat obtained from the sun for periods of time with no sunlight.
For example, Gemasolar power plant in Spain can store enough heat to produce electricity for extra 15 hours without any solar input ³.
The rest of the process of electricity generation is the same like at coal fueled power plants. The heat transferred by molten salt creates steam that runs steam turbines, which power a generator that produces electricity.
How do solar panels generate electricity?
The energy of sunlight is transformed directly into electricity thanks to the photovoltaic effect. Briefly described, this effect takes place when photons (tiny electromagnetic particles) of light are absorbed by a specific material, which in turn releases electrons from atoms. Released electrons are then available to do the electrical work, such as powering a light bulb.
Even though we cannot see the photovoltaic effect with our own eyes, we can have a look more closely at solar panels, where this process takes place.
What are solar panels?
Solar panels offer a familiar sight to many of us. Except the classic dark blue panels, scientists have been working in the past years on diverse alternatives, such as producing more aesthetically pleasing green-colored panels or cyanobacteria-powered bio-panels. But no matter how they look, solar panels serve as devices for capturing the energy of light.
One solar panel comprises of many smaller units called photovoltaic cells. Inside of these cells, the photovoltaic effect takes place. On average, one cell produces around 0.5 volts. Multiple cells are wired together in series to increase their output.
For example, a solar panel consisting of 36 interconnected cells generates around 18 volts.
Solar cells are made of semiconducting material, such as silicon, placed between two conducting materials to establish electric field. Commercially available cells contain phosphorus to provide negative electric charge and boron to give the positive charge. The electric field pushes electrons knocked by photons out of the silicon layer to metal plates on the sides of cells, from where they are transferred in a form of direct current⁴.
One of the most limiting factors of photovoltaic systems is the conversion rate of the sunlight into electricity, otherwise referred to as the efficiency. At most installations, this number remains between 15 to 18 percent. This means that over 80 percent of the sunlight falling on the solar panel is not transformed into power.
Where does the electricity go after?
When the solar energy is captured and collected by the solar cells, it is converted into direct current. The current flows into an inverter. The inverter is a device that transforms the direct current into alternate current, which is used to power our electronic devices⁵.
For any extra solar-generated electricity that is not used right away, there are two primary options and one hybrid option of their utilization. These are:
- On-grid systems
On-grid systems feed electricity back into the electrical grid. Electricity companies typically give you credits for the extra electricity that is fed back into the grid. You can use these credits to draw electricity from the grid on days when your solar array doesn’t produce any.
One drawback to this type of solar system is that when the grid is down, the system might not be utilized⁶.
- Off-grid systems
Off-grid systems are stand alone solar systems that rely on a battery to store solar energy for use during the night and at other times when there is more electricity needed than is being generated by the solar system.
- On-grid solar systems with a battery backup
On-grid solar systems with a battery backup feed solar energy-generated electricity back into the grid when the grid is operating, but in the event of a grid blackout, these systems will switch to an off-grid mode. In this off-grid mode, the backup battery is used to supply stored solar power, and the solar panels charge the battery⁶.
Both of these concepts show us that we are capable to satisfy our electricity demand by harvesting power from renewable resources. The only limitations arise from the available technology at the moment, which is an issue that can be tackled over the time, especially, if we focus our full attention on finding solutions to current obstacles, such as low efficiency of residential photovoltaic systems. Who knows, perhaps 21st century will be once known as the “Renewable Energy Revolution”.